The correct answer is C) green. The color of the signal flare sent up by a submarine to indicate that a torpedo has been fired in a training exercise is green. This is the standard color used to signify a training exercise or non-combat scenario, as opposed to other colors like red which would indicate a live, combat-related torpedo firing. The other answer choices are incorrect because white, yellow, and red flares are used to signal different types of emergencies or distress situations, not training exercises.